The present invention relates to a fuel cell comprising a solid polymer electrolyte used for portable power sources, electric vehicle power sources, domestic cogeneration systems, etc.
A fuel cell comprising a solid polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions, and a pair of electrodes formed on both surfaces of the polymer electrolyte membrane. The electrode usually comprises a catalyst layer which is composed mainly of carbon particles carrying a platinum group metal catalyst and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outer surface of the catalyst layer.
Moreover, gaskets or gas sealing materials are arranged on the outer periphery of the electrodes with the polymer electrolyte membrane therebetween so as to prevent a fuel gas and an oxidant gas from leaking out or prevent these two kinds of gases from mixing together. The gaskets are combined integrally with the electrodes and polymer electrolyte membrane beforehand. This is called xe2x80x9cMEAxe2x80x9d (membrane electrode assembly). Disposed outside the MEA are conductive separators for mechanically securing the MEA and for connecting adjacent MEAs electrically in series. The separators have a gas flow channel for supplying a reaction gas to the electrode surface and for removing a generated gas and an excess gas at a portion to come in contact with the MEA. Although the gas flow channel may be provided separately from the separators, grooves are usually formed on the surfaces of the separators to serve as the gas flow channel.
In order to supply the gas to such grooves, it is necessary to use a piping jig, called xe2x80x9cmanifoldxe2x80x9d, which branches out, depending on the number of the separators, into the grooves of the respective separators from a gas supply pipe. This type of manifold, directly connecting the gas supply pipe to the grooves of the separators, is specifically called xe2x80x9cexternal manifoldxe2x80x9d. There is also another type of manifold, called xe2x80x9cInternal manifoldxe2x80x9d, which has a more simple structure. In the internal manifold, the separators with the gas flow channel formed thereon are provided with through holes, called xe2x80x9cmanifold aperturexe2x80x9d, which are connected to the inlet and outlet of the gas flow channel, and the gas is supplied directly from the manifold apertures.
Since the fuel cell generates heat during operation, it needs cooling with cooling water or the like to keep good temperature conditions. Thus, a cooling section for flowing the cooling water therein is generally inserted between the separators for every one to three cells, and the cooling section is often formed by providing the backside of the separator with a cooling water flow channel. In a general structure of the fuel cell, the MEAs, separators and cooling sections, as described above, are alternately stacked to form a stack of 10 to 200 cells, and the resultant cell stack is sandwiched by end plates with a current collector plate and an insulating plate interposed between the cell stack and each end plate and is clamped with clamping bolts from both sides.
In such a polymer electrolyte fuel cell, the separators need to have a high conductivity, high gas tightness, and high corrosion resistance to oxidation/reduction reactions of hydrogen/oxygen. For such reasons, conventional separators are usually formed from carbon materials such as graphite and expanded graphite, and the gas flow channel is formed by cutting the surface of the separator or by molding in the case of expanded graphite separator.
The fuel cell produced in the above-described manner is supplied with the fuel gas, oxidant gas and cooling water to examine the performance of the fuel cell or of a unit cell of the fuel cell.
The prior art fuel cell, comprising the cell stack in which the MEA is disposed between two conventional conductive separators, poses a large problem resulting from the separators. Specifically, in such a fuel cell, the gasket arranged on the periphery of the MEA is pressed to fall into the gas flow channel of one of the two separators due to the clamping pressure of the fuel cell, thereby to form a clearance between the gasket of the MEA and the other separator. Such a clearance is liable to occur at the ends of the gas flow channel in the vicinity of the manifold apertures. Through the clearance, two kinds of gases mix with each other, resulting in deterioration of cell performance. Also, the mixing of the gasses may cause explosion or firing, thus inviting dangerous situations.
In view of the above problem of the prior art fuel cell, an object of the present invention is to provide a polymer electrolyte fuel cell free from mixing of two kinds of gases by improving separators.
Another object of the invention is to provide an improved separator that causes no mixing of two kinds of gasses.
The present invention is characterized in that in a conductive separator, the position at which the end of a gas flow channel is connected with a manifold aperture is changed in order to prevent mixing of the gases. Therefore, even if a gasket is pressed down toward the gas flow channel of the separator to form a clearance in the contacting portion of the gasket of an MEA and the separator in the vicinity of the manifold aperture in which a gas flows, the same kind of gas as the gas of the manifold aperture flows through the clearance, so that the mixing of the two kinds of gases does not occur in the present invention.
The present invention provides a polymer electrolyte fuel cell comprising:
a fuel cell stack comprising a plurality of conductive separators and a plurality of membrane electrode assemblies that are stacked with one of the conductive separators interposed therebetween, each of the membrane electrode assemblies comprising a polymer electrolyte membrane, and an anode and a cathode sandwiching the polymer electrolyte membrane;
a means for supplying a fuel gas to the anode; and
a means for supplying an oxidant gas to the cathode,
wherein the plurality of conductive separators comprise at least one separator comprising: a fuel gas inlet-side manifold aperture; a fuel gas outlet-side manifold aperture; a gas flow channel for supplying the fuel gas to the anode which is formed on an anode-side of the separator; an inlet-side through hole and an outlet-side through hole penetrating the separator which are formed at an inlet-side end and an outlet-side end of the gas flow channel for fuel gas; and an inlet-side connection groove and an outlet-side connection groove for connecting the inlet-side and outlet-side through holes with the fuel gas inlet-side manifold aperture and the fuel gas outlet-side manifold aperture, respectively, which are formed on a cathode-side of the separator.
It is preferable that the membrane electrode assembly further comprises a gasket covering an outer periphery of the anode and the cathode and that the gasket comprises a fuel gas inlet-side manifold aperture and a fuel gas outlet-side manifold aperture.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.